Wire with excellent suitability for drawing and process for producing the same

ABSTRACT

A hot-rolled wire rod excelling in wire drawability is provided, in which breakage can be suppressed even in heavy work from a large diameter. A hot-rolled wire rod contains C: 0.35 to 0.65% (percent by mass, hereinafter expressed as well), Si: 1.4 to 3.0%, Mn: 0.10 to 1.0%, Cr: 0.1 to 2.0%, P: 0.025% or less (exclusive of 0%), S: 0.025% or less (exclusive of 0%), N: 0.006% or less (exclusive of 0%), Al: 0.1% or less (exclusive of 0%), and O: 0.0030% or less (exclusive of 0%), with the remnant consisting of Fe and inevitable impurities; wherein the content of hydrogen in steel is 2.50 ppm (ppm by mass, hereinafter expressed as well) or less, and hardness (HV) is 460×C 0   0.1  or less (C 0  indicates the content of C (percent by mass) in a position of depth of D/4 (D: diameter of the wire rod)).

TECHNICAL FIELD

The present invention relates to wire rods that can be used formaterials of wire-drawing products such as steel cords, bead wire, PCsteel wire, and spring steel, and a method of manufacturing the wirerods; and particularly relates to hot-rolled wire rods excelling in wiredrawability, in which breakage can be suppressed even in heavy wiredrawing of wire rods having large diameters, and a manufacturing methodof the wire rods.

BACKGROUND ART

In the wire rods or the spring steel for wire drawing, wire drawabilityhas been improved by controlling microstructural factors, suppressingsegregation, or the like. For example, JP-A-11-199977 proposes thatpearlite nodule size, a center segregation level, and a lamellarinterval of a pearlite structure are controlled in order to improve wiredrawability (particularly, rod drawability) of wire rods.JP-A-2000-239797 proposes that mechanical properties of spring steel areappropriately adjusted to improve rod drawability of the spring steel.

For high alloy formation associated with increase in strength of aspring and the like, suppression of supercooled microstructures is alsorequired for the wire rods. Suppression of the supercooledmicrostructures can be achieved by manufacturing a wire rod having alarge wire diameter. However, the wire rod having the large wirediameter exhibits large work hardening due to heavy wire drawing, andfurthermore as initial wire diameter is increased, the wire drawingbecomes more difficult. Therefore, a wire rod having a large diameter isrequired to have higher wire drawability.

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

It is desirable to provide a hot-rolled wire rod excelling in wiredrawability, in which breakage can be suppressed even in heavy workusing a wire rod with a large diameter.

Means for Solving the Problem

A hot-rolled wire rod according to an embodiment of the inventioncontains C: 0.35 to 0.65% (percent by mass, hereinafter expressed aswell), Si: 1.4 to 3.0%, Mn: 0.10 to 1.0%, Cr: 0.1 to 2.0%, P: 0.025% orless (exclusive of 0%), S: 0.025% or less (exclusive of 0%), N: 0.006%or less (exclusive of 0%), Al: 0.1% or less (exclusive of 0%), and O:0.0030% or less (exclusive of 0%), with the remnant consisting of Fe andinevitable impurities; wherein the content of hydrogen in steel is 2.50ppm (ppm by mass, hereinafter expressed as well) or less, and hardness(HV) is 460×C₀ ^(0.1) or less (C₀ indicates the content of C (percent bymass) in a position of depth of D/4 (D: diameter of the wire rod)). The“hot-rolled wire rod” in the embodiment of the invention means an“as-hot-rolled wire rod”.

As a more preferable aspect of the hot-rolled wire rod according to theembodiment of the invention, (I) a wire rod is given, the rod havingaverage grain diameter (D_(ave)) of 20 μm or less, and maximum graindiameter (D_(max)) of 80 μm or less in a bcc-Fe grain of ametallographic structure, and/or a wire rod satisfying the followingequation (1) is given;C _(max) /C ₀≦1.20  (1)

(wherein C_(max) indicates the content of C (percent by mass) in aposition of depth of D/2 (D: diameter of the wire rod)), and C₀indicates the content of C (percent by mass) in the position of depth ofD/4).

Effectively, the hot-rolled wire rod of the embodiment of the inventionmay further contain the following as necessary: (A) Ni: 1% or less(exclusive of 0%) and/or Cu: 1.0% or less (exclusive of 0%), (B) atleast one element selected from a group including V: 0.30% or less(exclusive of 0%), Ti: 0.10% or less (exclusive of 0%), Nb: 0.1% or less(exclusive of 0%), and Zr: 0.10% or less (exclusive of 0%), (C) Mo: 1.0%or less (exclusive of 0%), (D) B: 50 ppm or less (exclusive of 0 ppm),and/or (E) at least one element selected from a group including Mg: 50ppm or less (exclusive of 0 ppm), Ca: 50 ppm or less (exclusive of 0ppm), and rare earth elements: 1.5 ppm or less (exclusive of 0 ppm);wherein properties of the wire rod are further improved depending on akind of components to be contained.

A manufacturing method according to an embodiment of the invention ispositioned as a useful method for manufacturing the hot-rolled wire rodhaving the described property, that is, excellent wire drawability. Afirst aspect of the manufacturing method of the embodiment of theinvention includes: performing heating in which a billet satisfyingrequirement of the composition (except for the hydrogen content) is heldat 500 to 730° C. for 60 min; heating the billet to 950 to 1250° C., andperforming hot rolling of the billet to make a wire rod at rollingtemperature (Tr) of 800° C. or more and finish rolling temperature (Tf)of 1150° C. or less; placing the hot-rolled wire rod on a cooling bed atcoiling temperature (TL) of 1020° C. or less; and cooling the wire at anaverage cooling rate (CR2) of 5° C./sec or less from the coilingtemperature (TL) to 500° C.

A second aspect of the manufacturing method of the embodiment of theinvention includes: performing heating in which a billet satisfyingrequirement of the composition (except for the hydrogen content) is heldat 500 to 730° C. for 60 min; heating the billet to 950 to 1250° C., andperforming hot rolling of the billet to make a wire rod at rollingtemperature (Tr) of 800° C. or more and finish rolling temperature (Tf)of 1150° C. or less; placing a hot-rolled wire rod on a cooling bed atcoiling temperature (TL) of 1020° C. or less; and cooling the wire at anaverage cooling rate (CR1) of 2° C./sec or more from the coilingtemperature (TL) to 730° C., and at an average cooling rate (CR2) of 5°C./sec or less from the coiling temperature (TL) to 500° C.

A third aspect of the manufacturing method of the embodiment of theinvention includes: performing homogenizing treatment in which a billetsatisfying requirement of the composition (except for the hydrogencontent) is held at 1250 to 1350° C. for 60 min; performing heating inwhich the billet is held at 500 to 730° C. for 60 min; heating thebillet to 950 to 1250° C., and performing hot rolling of the billet tomake a wire rod at rolling temperature (Tr) of 800° C. or more andfinish rolling temperature (Tf) of 1150° C. or less; placing thehot-rolled wire rod on a cooling bed at coiling temperature (TL) of1020° C. or less; and cooling the wire at an average cooling rate (CR1)of 2° C./sec or more from the coiling temperature (TL) to 730° C., andat an average cooling rate (CR2) of 5° C./sec or less from the coilingtemperature (TL) to 500° C.

A fourth aspect of the manufacturing method of the embodiment of theinvention includes: performing heating in which a billet satisfyingrequirement of the composition (except for the hydrogen content) is heldat 500 to 730° C. for 60 min; performing homogenizing treatment in whichthe billet is held at 1250 to 1350° C. for 60 min; heating the billet to950 to 1250° C., and performing hot rolling of the billet to make ahot-rolled wire rod at rolling temperature (Tr) of 800° C. or more andfinish rolling temperature (Tf) of 1150° C. or less; placing thehot-rolled wire rod on a cooling bed at coiling temperature (TL) of1020° C. or less to make a wire; and cooling the wire at an averagecooling rate (CR1) of 2° C./sec or more from the coiling temperature(TL) to 730° C., and at an average cooling rate (CR2) of 5° C./sec orless from the coiling temperature (TL) to 500° C.

Furthermore, an embodiment of the invention provides a method ofreducing the content of hydrogen in steel, including heating in which abillet is held at 500 to 730° C. for 60 min or more, the hydrogen havingadverse effect on wire drawability.

The inventors found that each of the contents of C, Si, Mn, Cr, P, S, N,Al and O in steel was specified, and the content of hydrogen in steelwas decreased, and hardness was controlled to be in a certain range orlower, thereby the hot-rolled wire rod excelling in wire drawability wasable to be provided, in which breakage was suppressed even in heavy workusing wire rods having large diameters.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relationship between hardness and C₀ (=thecontent of C (percent by mass) in a position of depth of D/4 (D:diameter of a wire rod)) of a wire rod obtained in an example.

BEST MODE FOR CARRYING OUT THE INVENTION

In the wire rod according to the embodiment of the invention, thecontent of hydrogen in steel is decreased to achieve excellent wiredrawability. It has been known so far that hydrogen adversely affectsthe steel under a stress loading condition lasting a long period of timewherein the hydrogen can sufficiently diffuse, for example, in the caseof delayed fracture, but it has been considered that hydrogen does notadversely affect the steel under a stress loading condition lasting acomparatively short period of time, such as in wire drawing. However,the inventors found that the hydrogen in steel, which had not beenregarded as a particular problem, had a large effect on wire drawabilityunder a heavy wire-drawing condition. When there are carbonitrides andthe like of an alloy element, which was added for increasing strength inthe wire rod, since they acts as hydrogen traps, the hydrogen content insteel is increased.

A reason for the adverse effect of the hydrogen in heavy wire drawing ispresumed to be because work hardening due to heavy work causes increasein strength which in turn increases hydrogen embrittlement sensibility,or hydrogen that has been fixed to a trap site is released from the siteby temperature rise due to heavy work, and contributes to theembrittlement. However, the embodiment of the invention is not limitedto such presumption.

To sufficiently suppress the breakage even in heavy work, the content ofhydrogen in steel of the hot-rolled wire rod needs to be 2.50 ppm orless. The content of hydrogen in steel is preferably 2 ppm or less, andmore preferably 1.5 ppm or less.

The content of hydrogen in steel can be measured using APIMS(Atmospheric Pressure Ionization Mass Spectrometer). A value of “thecontent of hydrogen in steel” in the embodiment of the invention is madeby sampling a disk-like sample (thickness: 2 mm) by cutting a wire rod,then measuring the total content of hydrogen evaluated from the samplefrom room temperatures to 350° C. under a condition of a heating rate of10 K/min using APIMS.

As a result of further investigation, the inventors found that there wasa certain relationship between wire drawability and hardness of a wirerod, and when initial hardness of the wire rod was high, breakage wasapt to occur during wire drawing. The reason for this is considered tobe because when the initial hardness is high, fracture sensitivity isincreased since work hardening becomes more significant, or effect ofheat due to work is significant. However, the embodiment of theinvention is not limited to such presumption.

Hardness of a wire rod is mainly affected by the content of C and astructure of the wire rod. Generally, as the content of C is increased,or an amount of a martensite structure as the supercooled microstructureis increased, hardness is increased. The microstructure of the wire rodaffects wire drawability similarly as hardness. Specifically, it isconsidered that the larger the amount of martensite, the more easilybreakage occurs in a wire rod.

As hereinbefore, wire drawability of a wire rod (breakability) isaffected not only by hardness, but also by its microstructure.Therefore, even in wire rods having the same hardness, breakage easilyoccur in a wire rod having a low content of C and a large amount ofmartensite structure compared with a wire rod having a high content of Cand a large amount of ferrite-pearlite structure. Accordingly, it can besaid that breakage hardly occurs in a wire rod having the high contentof C compared with a wire rod having the low content of C if they havethe same hardness, in addition, it can be considered that a referencevalue (maximum value) of hardness allowed in a wire rod having excellentwire drawability can be set high in the wire rod having a high contentof C.

Based on consideration as above, still in the light of themicrostructure, “hardness (HV) of 460×C₀ ^(0.1) or less (C₀ indicatesthe content of C (percent by mass) in a position of depth of D/4 (D:diameter of the wire rod)) was determined as a requirement of hardness.The requirement of hardness≦460×C₀ ^(0.1) is obtained in the followingway.

In the following embodiments, when data of “C₀” and “hardness” of a wirerod (comparative example, black circles in FIG. 1), of which the wiredrawability is considered to be reduced due to high hardness, aresubjected to power approximation, a curve in a solid line as shown inFIG. 1 is obtained (approximate expression: hardness=466.06×C₀ ^(0.10)(R²=0.62)).

In this approximate expression (hardness=466.06×C₀ ^(0.10)), as a valueof C₀ is increased, a value of hardness is also increased, andconversely as the value of C₀ is decreased, the value of hardness isalso decreased. Accordingly, the inventors considered the approximateexpression as an expression indicating a reference value (maximum value)of hardness of a wire rod that is easily broken in considerationincluding the microstructure. In FIG. 1, a region of a curve in a brokenline (hardness=460×C₀ ^(0.10)) or lower, which is below the curve in thesolid line (approximate curve of the comparative example), that is, aregion of “hardness≦460×C₀ ^(0.10)” was determined as a range ofhardness to be satisfied by the wire rod of the embodiment of theinvention. A preferable range is “hardness≦450×C₀ ^(0.10)” (a region ofa curve in a chain line or lower in FIG. 1), and a more preferable rangeis “hardness≦440×C₀ ^(0.10)” (a region of a curve in a dot line or lowerin FIG. 1).

When the structure is not considered, it is considered that as hardnessis decreased, wire drawability is improved. Accordingly, in theembodiment of the invention, a maximum value of hardness (HV) of thewire rod is preferably 420, more preferably 410 or less, and furtherpreferably 400 or less.

The value of “hardness” in the embodiment of the invention is a simplearithmetic mean value of values obtained by cutting a wire rod in alateral cross section to prepare at least three samples per wire rod,then measuring hardness at four points or more in positions of depth ofD/4 of each sample by a Vickers hardness tester (load of 1 kgf).

Among the hot-rolled wire rods of the embodiment of the invention, awire rod is preferable, which has an average grain diameter (D_(ave)) of20 μm or less and a maximum grain diameter (D_(max)) of 80 μm or less ina bcc-Fe grain of a metallographic structure. This is because it wasfound that start points of breakage or working defects during wiredrawing were easily generated in the case of coarse grains, andfurthermore even if an average value of grain diameter was made small,when there were some coarse grains, breakage easily occurred. As both ofthe average grain diameter (D_(ave)) and the maximum grain diameter(D_(max)) are smaller, wire drawability is improved. More preferably,the average grain diameter (D_(ave)) is 15 μm or less, and the maximumgrain diameter (D_(max)) is 60 μm or less. Values of the average graindiameter (D_(ave)) and the maximum grain diameter (D_(max)) in theembodiment of the invention are measuring values in the center of a wirediameter of a wire rod.

The values of the average grain diameter (D_(ave)) and the maximum graindiameter (D_(max)) in the embodiment of the invention are valuesmeasured in the following way using a SEM/EBSP (Electron Back Scatterdiffraction Pattern) method.

First, a sample 10 mm in length is taken from a wire rod by wet cutting,then as sample preparation for EBSP measurement, wet polishing, buffing,and chemical polishing are performed so that a sample is prepared, inwhich strain and irregularity due to polishing are reduced to theutmost. At that time, the polishing is performed such that anobservation surface corresponds to a center of wire diameter in avertical section of the wire rod. Using an obtained sample, measurementis performed with the center of wire diameter of the wire rod as an EBSPmeasurement point. At that time, a measurement step is set to be 0.5 μmor less such that a measurement area of each wire rod is 60,000 μm² ormore. After measurement, crystal orientation is analyzed, in whichmeasuring results having an average CI (Confidence Index) value of 0.3or more are used to improve reliability of the analysis.

Analytical results (boundary map) are collected assuming that a regionenclosed by a boundary line having difference in azimuth of 10 degreesor more by analysis of the bcc-Fe crystal orientation is the “grain” inthe embodiment of the invention. In the obtained boundary map, an areaof an individual region (crystal unit) enclosed by the boundary line isobtained using an image analysis software “Image-Pro” (manufactured byADVANSOFT Ltd.), then circle equivalent diameter (diameter) iscalculated from the area as the grain diameter of an individual grain.The measurement is performed for at least three samples, and the averagegrain diameter (D_(ave)) as the number average diameter, and the maximumgrain diameter (D_(max)) are calculated based on all measurement data.

In the hot-rolled wire rod according to the embodiment of the invention,to further improve the wire drawability, segregation of C is preferablycontrolled such that the following equation (1) is satisfied:C _(max) /C ₀≦1.20  (1)

(wherein C_(max) indicates the content of C (percent by mass) in aposition of depth of D/2 (D: diameter of the wire rod)), and C₀indicates the content of C (percent by mass) in the position of depth ofD/4).

This is because when the segregation of C is excessive, wire drawabilitymay be reduced because work hardening during wire drawing may becomeuneven within a wire rod, or voids are easily generated in a segregationsite of C. The C_(max)/C₀ of the wire rod in the embodiment of theinvention is preferably 1.15 or less, and more preferably 1.10 or less.

The embodiment of the invention adopted the content of C (percent bymass) in the position of depth of D/2 (D: diameter of the wire rod) as avalue of C_(max). This is because segregation of carbon is significantin the central portion of the wire rod. Furthermore, the embodimentadopted the content of C (percent by mass) in the position of depth ofD/4 as a value of C₀. This is for avoiding effect of a decarburized sitein a surface and the segregation site of C in the center. The value ofthe C_(max) or C₀ in the embodiment of the invention is measured by acombustion infrared absorption method using a powdered sample taken fromthe position of depth of D/2 or D/4, respectively.

The embodiment of the invention specifies a chemical composition inaddition to the content of hydrogen in steel and hardness of thehot-rolled wire rod. This is because when each chemical component is notwithin an appropriate range, the wire drawability is reduced.Hereinafter, chemical components of the wire rod are described.

[C Content: 0.35 to 0.65%]

C is an element affecting strength of steel materials, and as the Ccomponent is increased, the strength is increased. The C content of atleast 0.35% is necessary to use the wire rod for high-strength springs.Preferably, the minimum C content is 0.40%. However, since an excessiveC content may reduce the wire drawability, a maximum C content isspecified as 0.65%. More preferably, the maximum C content is 0.60%.

[Si Content: 1.4 to 3.0%]

Si is an element effective for improving sag resistance necessary forsprings. The Si content of at least 1.4% is necessary to use the wirerod of the embodiment of the invention for high-strength springs. Theminimum Si content is preferably 1.6%, and more preferably 1.8%.However, since Si accelerates decarburization, an excessive Si contentmay cause breakage to easily occur during the wire drawing. Thus, amaximum Si content is specified as 3.0%. The maximum Si content ispreferably 2.5%, and more preferably 2.2% or less.

[Mn Content: 0.10 to 1.0%]

Mn is used for a deoxidizing element, and is a useful element to formMnS to detoxify S which is a harmful element in the steel. Tosufficiently exhibit these advantageous effects, the Mn content needs tobe 0.10% or more. A minimum Mn content is preferably 0.15%, and morepreferably 0.2% or more. However, when the Mn content is excessive, asegregation band is formed, which reduces the wire drawability, inaddition, a supercooled microstructure, which is not preferable for wiredrawing, is easily formed. Thus, a maximum Mn content was specified as1.0%. The maximum Mn content is preferably 0.85%, and more preferably0.75% or less.

[Cr Content: 0.1 to 2.0%]

Cr is effective for securing strength of the wire rod after tempering.Moreover, it has an advantage of improving corrosion resistance, and isan important element for suspension springs requiring corrosiondurability. A minimum Cr content was specified as 0.1% to sufficientlyexhibit these advantages. The minimum Cr content is preferably 0.15%,and more preferably 0.2% or more. However, when the Cr content isexcessive, segregation easily occurs or the supercooled microstructureis easily formed, reducing the wire drawability. Thus, a maximum Crcontent is specified as 2.0%. The maximum Cr content is preferably 1.8%,and more preferably 1.6% or less.

[P Content: 0.025% or Less (Exclusive of 0%)]

The content of P is preferably low, because it reduces the wiredrawability of the wire rod. Accordingly, the P content is 0.025% orless, preferably 0.020% or less, and more preferably 0.015% or less.

[S Content: 0.025% or Less (Exclusive of 0%)]

The content of S is preferably low because it reduces the wiredrawability of the wire rod. Accordingly, the S content is 0.025% orless, preferably 0.020% or less, and more preferably 0.015% or less.

[N content: 0.006% or less (exclusive of 0%)]

N in a state of dissolved nitrogen may reduce the wire drawability.Thus, a maximum N content is specified as 0.006%. The maximum N contentis preferably 0.004%, and more preferably 0.003% or less. However, whena wire rod contains an element forming nitrides, such as Al or Ti, N mayeffectively work for formation of a fine structure. Accordingly, aminimum N content is preferably 0.0015%, and more preferably at least0.0020%.

[Al Content: 0.1% or Less (Exclusive of 0%)]

Al is added mainly as a deoxidizing element. Moreover, Al forms AlN tofix N to be harmless, in addition, it contributes to formation of a finestructure. For fixing N, Al is preferably contained in the content ofmore than two times as much as the N content. Desirably, the content ofAl is preferably more than 0.0030%, and more preferably more than0.0040%. However, since Al accelerates decarburization, particularly inspring steels containing a large amount of Si, the excessive Al contentis not preferable. Thus, a maximum Al content is specified as 0.1%. Themaximum Al content is preferably 0.07%, more preferably 0.05% or less,and further preferably 0.03% or less.

[O Content: 0.0030% or Less (Exclusive of 0%)]

When the content of oxygen in steel is increased, since coarse oxidesare formed, reducing the wire drawability, the content is preferablysmall. Accordingly, the maximum O content is specified as 0.0030%. Themaximum O content is preferably 0.0020%, and more preferably 0.0015% orless.

A basic composition of the wire rod of the embodiment of invention is asabove, and the remnant is substantially Fe. However, the wire rod isobviously allowed to contain inevitable impurities introduced dependingon conditions of raw materials, other materials, and manufacturingequipment. Furthermore, the wire rod of the embodiment of invention maycontain the following optional elements as necessary.

[Ni Content: 1% or Less]

Ni has an advantage of suppressing superficial decarburization, inaddition, an advantage of improving corrosion resistance. Tosufficiently exhibit the advantages, the content of Ni is preferably atleast 0.1%, and more preferably at least 0.2%, as necessary. However,when the Ni content is excessive, the supercooled microstructure iseasily formed, consequently the wire drawability is reduced.Accordingly, when Ni is contained, the Ni content is preferably 1% orless, more preferably 0.8% or less, and further preferably 0.6% or less.

[Cu Content: 1.0% or Less]

Cu also has the advantage of suppressing superficial decarburization,and in addition, the advantage of improving corrosion resistance,similar to Ni. To sufficiently exhibit the advantages, the content of Cuis preferably at least 0.1%, and more preferably at least 0.2%, asnecessary. However, when the Cu content is excessive, a supercooledmicrostructure is easily formed, and consequently, the wire drawabilityis reduced. Moreover, cracks may occur during hot working. Accordingly,when Cu is contained, the Cu content is preferably 1.0% or less, morepreferably 0.8% or less, and further preferably 0.6% or less.

Ni and Cu are common in that they contribute to suppressing thesuperficial decarburization and improving corrosion resistance.Therefore, the hot-rolled wire rod preferably contains at least one ofNi and Cu in the amount stated above.

[V Content: 0.30% or Less]

V mainly forms carbonitrides with C and N and thus contributes toformation of a fine structure. To sufficiently exhibit the advantage,the content of V is preferably at least 0.01%, and more preferably atleast 0.05%, as necessary. However, when the V content is excessive, thewire drawability is reduced. Accordingly, when V is contained, the Vcontent is preferably 0.30% or less, more preferably 0.2% or less, andfurther preferably 0.15% or less.

[Ti Content: 0.10% or Less]

Ti forms carbonitrides or sulfides with C and N, or S, and thus works todetoxify N and S. Moreover, Ti carbonitrides have an advantage ofcontributing to formation of the fine structure. To sufficiently exhibitthe advantages, the content of Ti is preferably 0.01% or more, asnecessary. From a viewpoint of fixing N, the Ti content is preferablymore than three and half times the N content. However, when the Ticontent is excessive, coarse carbonitrides are formed, and consequentlythe wire drawability may be reduced. Accordingly, when Ti is contained,the Ti content is preferably 0.10% or less, more preferably 0.07% orless, and further preferably 0.05% or less.

[Nb Content: 0.1% or Less]

Nb forms carbonitrides with C and N and thus contributes to formation ofthe fine structure. To sufficiently exhibit the advantage, the contentof Nb is preferably at least 0.01%, and more preferably at least 0.03%,as necessary. However, when the Nb content is excessive, coarsecarbonitrides are formed, and consequently the wire drawability isreduced. Accordingly, when Nb is contained, the Nb content is preferably0.1% or less, more preferably 0.07% or less, and further preferably0.05% or less.

[Zr Content: 0.10% or Less]

Zr forms carbonitrides and thus contributes to formation of the finestructure. To sufficiently exhibit the advantage, the content of Zr ispreferably 0.01% or more, and more preferably 0.02% or more, asnecessary. However, when the Zr content is excessive, coarsecarbonitrides are formed, and consequently the wire drawability isreduced. Accordingly, when Zr is contained, the Zr content is preferably0.10% or less, more preferably 0.07% or less, and further preferably0.05% or less.

V, Ti, and Nb are common in that they contribute to formation of thefine structure by forming carbonitrides. The hot-rolled wire rodpreferably contains at least one of V, Ti, and Nb of the amount statedabove.

[Mo Content: 1.0% or Less]

Mo forms carbonitrides with C and N, and concentrates in cementite andthus contributes to securing strength. To sufficiently exhibit theadvantages, the content of Mo is preferably at least 0.1%, and morepreferably at least 0.2%, as necessary. However, when the Mo content isexcessive, the supercooled microstructure is easily formed, andconsequently the wire drawability is reduced. Accordingly, when Mo iscontained, the Mo content is preferably 1.0% or less, more preferably0.7% or less, and further preferably 0.5% or less.

[B Content: 50 ppm or Less]

B forms nitrides and thus detoxifies N. To sufficiently exhibit theadvantage, the content of B is preferably at least 1 ppm, morepreferably 3 ppm or more, and further preferably at least 5 ppm, asnecessary. However, when the B content is excessive, since coarsecarbonitrides and the supercooled microstructure are formed, the wiredrawability is reduced. Accordingly, when B is contained, the B contentis preferably 50 ppm or less, more preferably 40 ppm or less, andfurther preferably 30 ppm or less.

[Mg Content: 50 ppm or Less]

Mg has an advantage of softening oxides and thus improving the wiredrawability. To sufficiently exhibit the advantage, the content of Mg ispreferably at least 0.1 ppm, more preferably at least 1 ppm, and furtherpreferably at least 10 ppm, as necessary. However, when the Mg contentis excessive, properties of the oxides are changed, and consequently thewire drawability may be rather reduced. Accordingly, when Mg iscontained, the Mg content is preferably 50 ppm or less, and morepreferably 40 ppm or less.

[Ca Content: 50 ppm or Less]

Ca has an advantage of softening oxides and thus improving the wiredrawability. To sufficiently exhibit the advantage, the content of Ca ispreferably at least 0.1 ppm, more preferably at least 1 ppm, and furtherpreferably at least 10 ppm, as necessary. However, when the Ca contentis excessive, properties of the oxides are changed, and consequently thewire drawability may be rather reduced. Accordingly, when Ca iscontained, the Ca content is preferably 50 ppm or less, and morepreferably 40 ppm or less.

Mg and Ca are common in that they improve the wire drawability bysoftening oxides. Therefore, the hot-rolled wire rod preferably containsat least one of Mg and Ca in the amount stated above.

[Content of Rare Earth Elements: 1.5 ppm or Less]

Rare earth elements (sometimes abbreviated as “REM”) have an advantageof softening oxides and thus improving the wire drawability. Tosufficiently exhibit the advantage, the content of REM is preferably atleast 0.1 ppm, as necessary. However, when the content of REM isexcessive, properties of the oxides are changed, and consequently thewire drawability may be rather reduced. Accordingly, when REM iscontained, the content of REM is preferably 1.5 ppm or less, and morepreferably 0.5 ppm or less. Preferable elements among REM are La, Ce, Prand Nd, and one or at least two of them can be used.

The hot-rolled wire rod satisfying requirements of the content ofhydrogen in steel and the hardness (preferably, requirement of the graindiameter in addition) can be manufactured by: performing heating inwhich a billet satisfying the requirement of the composition is held at500 to 730° C. for 60 min; heating the billet to 950 to 1250° C., andperforming hot rolling of the billet to make a wire rod at rollingtemperature (Tr) of 800° C. or more and finish rolling temperature (Tf)of 1150° C. or less; placing the hot-rolled wire rod on a cooling bed atcoiling temperature (TL) of 1020° C. or less to make a wire; and coolingthe wire at an average cooling rate (CR2) of 5° C./sec or less from thecoiling temperature (TL) to 500° C. (preferably at an average coolingrate (CR1) of 2° C./sec or more from the coiling temperature (TL) to730° C.). Hereinafter, each of steps of this manufacturing method isdescribed.

Hydrogen may enter steel during a manufacturing process of the steel(wire rod). In particular, since the hot-rolled wire rod of theembodiment of the invention, and the billet for obtaining the wire rodcontain various alloy elements, carbonitrides or nonmetal inclusions ofthem may form hydrogen trap sites, thereby hydrogen easily accumulatesin steel. Since the hydrogen traps are robust, hydrogen is hardlyreleased from the trap under a condition of the normal temperature. Theinventors evaluated trap capability of the hydrogen trap sites, and as aresult, found that the steel was acceptably subjected to heating inwhich it was held at a temperature of 500° C. or more for 60 min or morein order to effectively decrease the content of hydrogen in steel.However, they further found that when the billet was excessively heatedto high temperature at which austenite was formed, since hydrogen waseasily dissolved in austenite compared with ferrite, hydrogen was ratherhard to be released.

Accordingly, to efficiently decrease the content of hydrogen in steel ofthe wire rod, a billet before rolling can be heated at 500 to 730° C.,preferably 550 to 700° C., for 60 min or more, preferably for 120 min ormore. The heating before rolling is important as a step in a method ofmanufacturing a hot-rolled wire rod excelling in wire drawability, anduseful as a method of decreasing hydrogen in steel of the hot-rolledwire rod. The heating may be performed in either of an inline that isthe same as a rolling line and an offline separated from the rollingline.

Then, the billet satisfying the requirement of the composition is heatedto the range of 950 to 1250° C., preferably 1000 to 1200° C., andsubjected to hot rolling at the rolling temperature (Tr) of at least800° C., preferably at least 850° C., and more preferably at least 900°C., and the finish rolling temperature (Tf) of 1150° C. or less, andpreferably 1100° C. or less. In both cases of extremely low and highheating temperature before rolling, decarburization occurs in thesurface of the wire rod. When the rolling temperature is less than 800°C., possibility of decarburization is increased. When the finish rollingtemperature is a high temperature of more than 1150° C., hardenabilityis increased due to growth of austenite grains, causing increase inhardenability, and consequently, strength of the wire rod may beexcessively increased.

It is recommended that the wire rod is placed on the cooling bed at thecoiling temperature (TL) of 1020° C. or less, preferably 980° C. orless, and more preferably 950° C. or less. This is because when thecoiling temperature exceeds 1020° C., austenite grain size is enlarged.It is necessary to decrease hardness of the wire rod that the wire rodis cooled at the average cooling rate (CR2) of 5° C./sec or less fromthe coiling temperature (TL) to 500° C. Furthermore, by such slowcooling from the coiling temperature (TL) to 500° C., the content ofhydrogen in steel can be further decreased. CR2 is preferably 4° C./secor less, and more preferably 3° C./sec or less.

However, to form a fine structure due by inhibiting growth of austenitegrains and decrease in hardness, it is effective that the cooling rateCR1 from the coiling temperature (TL) to 730° C. is preferably at least2° C./sec, more preferably at least 5° C./sec, and further preferably atleast 8° C./sec.

To suppress segregation of C so that C_(max)/C₀ is 1.20 or less, soakingis added to the manufacturing method, in which the billet satisfying therequirement of the composition is held at 1250 to 1350° C., preferably1280 to 1310° C., for 60 min or more, preferably for 120 min or more,before rolling. The soaking may be performed in either of an inline thatis the same as the rolling line and an offline separated from therolling line. Moreover, it may be performed before or after the heatingfor decreasing the content of hydrogen in steel.

However, to further decrease the content of hydrogen in steel, it ispreferable that the soaking is performed to eliminate the segregationband before the heating. Moreover, it is preferable that the soakingrequiring high temperature is performed in an offline different from therolling line, and the heating for decreasing the content of hydrogen insteel is performed in the inline that is the same as the rolling line,in addition, from a viewpoint of equipment, it is preferable that firstthe soaking is performed before the heating.

In the embodiment of the invention, wire diameter of the hot-rolled wirerod is not particularly limited. However, the wire diameter ispreferably large to suppress formation of the supercooledmicrostructure. The wire rod of the embodiment of the invention isexcellent in wire drawability, therefore breakage can be effectivelysuppressed even if the rod is subjected to heavy work from a largediameter. Accordingly, a minimum wire diameter is preferably 8 mm, morepreferably at least 10 mm, and further preferably at least 12 mm. On theother hand, since excessive large wire diameter causes difficulty inwire drawing, a maximum wire diameter is preferably 25 mm, morepreferably 20 mm, and further preferably 18 mm.

Embodiment

Hereinafter, while the invention will be described more specificallywith an embodiment, the invention is not limited by the followingembodiment, and it can be obviously practiced by being appropriatelymodified within a scope adaptable to the purport described before andafter, and any of such modifications may be covered within a technicalscope of the invention.

[Manufacturing of Wire Rods]

Steel materials having chemical compositions listed in Tables 1-1 to 1-2(the remnant: iron and inevitable impurities) were ingoted, and shapedinto billets 155 mm square. Next, soaking, heating, hot rolling,coiling, and cooling were performed in order under conditions listed inTables 2-1 to 2-3, and consequently, hot-rolled wire rods 8.0 to 18 mmin wire diameter were manufactured.

TABLE 1-1 Mass percent Steel type No. C Si Mn Cr P S N Al O A1 0.38 1.780.20 1.05 0.008 0.008 0.0041 0.0300 0.0019 A2 0.40 2.09 0.85 1.83 0.0030.002 0.0032 0.0321 0.0018 A3 0.42 2.71 0.94 1.92 0.002 0.002 0.00280.0003 0.0010 A4 0.44 1.92 0.18 1.00 0.008 0.007 0.0039 0.0310 0.0012 A50.47 2.05 0.79 0.18 0.015 0.016 0.0035 0.0280 0.0011 A6 0.50 2.01 0.621.21 0.021 0.020 0.0028 0.0300 0.0011 A7 0.50 2.01 0.62 1.21 0.027 0.0200.0028 0.0300 0.0011 A8 0.50 2.01 0.39 1.83 0.013 0.014 0.0032 0.03000.0008 A9 0.50 2.18 0.18 1.20 0.005 0.006 0.0028 0.0320 0.0005 A10 0.512.40 0.18 1.02 0.004 0.005 0.0030 0.0310 0.0005 A11 0.52 2.41 0.18 1.040.004 0.006 0.0032 0.0290 0.0009 A12 0.55 1.81 0.77 0.70 0.013 0.0090.0041 0.0003 0.0012 A13 0.55 2.32 0.92 1.88 0.003 0.003 0.0033 0.00150.0011 A14 0.57 1.41 0.76 0.70 0.016 0.016 0.0039 0.0320 0.0014 A15 0.580.19 0.90 0.85 0.014 0.013 0.0066 0.5210 0.0034 A16 0.61 3.12 1.21 0.200.005 0.004 0.0030 0.0005 0.0007 A17 0.61 1.47 0.53 0.54 0.012 0.0070.0029 0.0270 0.0010 A18 0.63 1.62 0.51 0.72 0.008 0.008 0.0030 0.03100.0011 A19 0.70 0.18 0.50 2.12 0.005 0.004 0.0025 0.0015 0.0010 A20 0.810.20 0.07 0.015 0.026 0.0027 0.0210 0.0022

TABLE 1-2 Steel Mass percent PPM by mass type No. Ni Cu Mo V Ti Nb Zr MgCa REM B A1 0.53 0.22 0.0 0.168 0.065 0.2 2.7 1.0 A2 A3 A4 0.50 0.25 0.00.155 0.068 0.1 1.8 1.0 A5 0.30 0.28 0.0 0.156 0.072 0.1 1.9 0.1 A6 0.020.01 0.6 0.051 0.008 A7 0.02 0.01 1.2 0.080 0.051 A8 0.01 0.02 0.0790.048 A9 0.40 0.39 0.070 35.0 34.0 23.0 A10 0.60 0.58 0.050 35.0 38.022.0 A11 0.61 0.57 0.050 1.0 A12 0.03 0.007 0.072 0.1 1.2 0.1 A13 A140.02 0.03 0.020 0.1 1.3 1.0 A15 0.7 A16 1.22 1.09 0.2 2.5 0.1 A17 0.168A18 0.075 0.059 A19 0.321 0.105 A20 0.110 0.1 0.8 55.0 REM: the totalcontent of La, Ce, Pr and Nd

TABLE 2-1 Rolling Cooling Minimum Finish Cooling Cooling Soaking HeatingHeating rolling rolling Coiling rate rate Steel Wire rod TemperatureTime Temperature Time temperature temperature temperature temperatureCR1 CR2 type No. No. ° C. minutes ° C. minutes ° C. ° C. ° C. ° C. °C./sec ° C./sec A1 A1-1 — — — — 1240 950 1080 990 12.0 3.5 A1-2 — — 600120 1240 950 1080 990 12.0 3.1 A1-3 — — 700 120 1240 950 1080 990 12.23.7 A1-4 — — 700 120 1220 950 1170 1050 12.2 6.1 A1-5 1280 60 550 1201220 950 1045 960 7.1 2.5 A1-6 1280 60 600 60 1220 950 1045 960 9.2 2.9A1-7 1280 60 700 60 1220 950 1045 960 6.3 2.2 A1-8 1280 60 700 60 1220950 1020 960 3.7 1.4 A2 A2-1 1310 60 600 60 1230 1000 1070 990 4.2 1.3A3 A3-1 1310 60 600 60 1230 1000 1070 990 4.0 1.1 A4 A4-1 — — 600 201220 950 1045 950 13.0 5.5 A4-2 — — 600 60 1220 950 1045 950 8.8 2.6A4-3 — — 600 60 1220 950 1045 950 7.3 2.5 A4-4 — — 700 60 1220 950 1045950 12.0 3.7 A4-5 1310 60 600 60 1200 920 1080 980 1.0 1.2 A4-6 1310 60600 60 1200 920 1080 980 16.0 2.7

TABLE 2-2 Rolling Cooling Minimum Finish Cooling Cooling Soaking HeatingHeating rolling rolling Coiling rate rate Steel Wire rod TemperatureTime Temperature Time temperature temperature temperature temperatureCR1 CR2 type No. No. ° C. minutes ° C. minutes ° C. ° C. ° C. ° C. °C./sec ° C./sec A5 A5-1 1260 60 550 20 1200 950 1045 980 15.2 6.8 A5-21260 60 550 40 1200 950 1045 980 12.8 5.9 A5-3 1260 60 550 120 1200 9501045 980 0.5 2.8 A5-4 1260 60 600 60 1200 950 1045 950 6.7 1.8 A5-5 126060 600 60 1200 950 1045 950 3.8 1.7 A6 A6-1 1310 60 — — 1170 920 1020925 12.2 2.3 A6-2 1310 60 700 60 1170 920 1020 925 12.5 2.0 A7 A7-1 128060 — — 1170 920 1020 925 12.1 2.9 A7-2 1280 60 700 60 1170 920 1020 92512.0 3.7 A8 A8-1 1280 60 — — 1200 920 1000 925 2.7 1.5 A8-2 1280 60 72060 1200 920 1000 925 2.5 1.4 A9 A9-1 — — — — 1200 920 1000 925 2.5 1.8A9-2 — — 650 120 1200 920 1000 925 2.4 1.7 A10 A10-1 1280 60 650 1201150 900 990 900 10.0 1.3 A11 A11-1 1280 60 650 120 1150 900 990 900 9.71.4

TABLE 2-3 Rolling Cooling Minimum Finish Cooling Cooling Soaking HeatingHeating rolling rolling Coiling rate rate Steel Wire rod TemperatureTime Temperature Time temperature temperature temperature temperatureCR1 CR2 type No. No. ° C. minutes ° C. minutes ° C. ° C. ° C. ° C. °C./sec ° C./sec A12 A10-1 1260 60 700 60 1050 850 1000 900 11.8 1.2 A13A13-1 1310 60 600 60 1220 930 1030 990 4.5 1.4 A13-2 1310 60 600 60 1220930 1030 990 10.1 2.1 A13-3 1310 60 600 60 1220 930 1030 990 14.3 3.2A14 A14-1 1260 60 700 60 1000 850 900 880 11.2 1.2 A15 A15-1 1260 60 70060 1000 850 900 880 10.8 1.5 A16 A16-1 1260 60 700 60 1150 900 950 92510.2 1.9 A17 A17-1 — — — — 1150 900 1050 925 8.9 2.2 A17-2 — — 400 601150 900 1050 925 9.4 2.4 A17-3 — — 600 60 1150 900 1080 925 9.0 2.0A17-4 — — 600 60 1100 870 1080 925 14.3 5.9 A17-5 — — 700 60 1100 8701080 900 15.7 3.1 A17-6 — — 700 180 1100 870 1080 900 15.0 2.7 A17-7 — —700 180 1100 870 1080 900 15.0 0.4 A18 A18-1 — — 700 180 1150 900 1000925 15.7 1.8 A19 A19-1 1280 60 700 60 1150 900 1050 900 9.5 2.2 A20A20-1 1280 60 700 60 1150 900 1050 900 10.3 2.4

[Content of Hydrogen in Steel]

As the content of hydrogen in steel, the total hydrogen contentevaluated from a disk-like sample (thickness: 2 mm) from roomtemperatures to 350° C. under a condition of heating temperature of 10K/min was measured using APIMS. Results are shown in Tables 3-1 to 3-3.

[Hardness]

The wire rods were cut in lateral cross sections to prepare threesamples per wire rod, and at a position of depth of D/4 of each sample,hardness was measured at four points by a Vickers hardness tester (load:1 kgf), and the simple arithmetic mean of obtained values was obtained,so that hardness of each wire rod was calculated. Results are shown inTables 3-1 to 3-3.

A graph showing a relationship between C₀ (C₀ indicates the C content(mass percent) at the position of depth of D/4 (D: diameter of wirerod)) and hardness of each wire rod is represented as FIG. 1. In FIG. 1,black circles (beyond the hardness range of the present invention) are aplot of data of wire rods A1-4, A2-1, A3-1, A3-2 and A14-4; blacksquares (beyond the composition range of the present invention) are aplot of wire rod data obtained from steel types A5, A12, A13, A16 andA17; black triangles (beyond the hydrogen content range of the presentinvention) are a plot of data of wire rods A1-1, A4-1, A6-1, A7-1, A14-1and A14-2; and white circles (inventive example) are a plot of otherwire rod data.

The data of the wire rods A1-4, A2-1, A3-1, A3-2 and A14-4 weresubjected to power approximation, consequently an approximate expressionof hardness=466.06×C₀ ^(0.10) (R²=0.62) was obtained. Such anapproximate curve is also shown in FIG. 1 by a solid line. In FIG. 1,similarly, an approximate curve of 460×C₀ ^(0.10) is shown in a brokenline, an approximate curve of 450×C₀ ^(0.10) is shown in a dashed line,and an approximate curve of 440×C₀ ^(0.10) is shown in a dot line.

[Average Grain Diameter (D_(ave)) and Maximum Grain Diameter (D_(max))]

A sample 10 mm in length was taken from each of the wire rods by wetcutting, then as sample preparation for EBSP measurement, wet polishing,buffing, and chemical polishing were performed so that a sample wasprepared, in which strain and irregularity due to polishing were reducedto the utmost. At that time, the polishing was performed such that anobservation surface corresponds to a center of wire diameter in avertical section of the wire rod. Using an obtained sample, measurementwas performed with the center of wire diameter of the wire rod as anEBSP measurement point. At that time, a measurement step was set to be0.5 μm or less such that a measurement area of each wire rod was 60,000μm² or more. After measurement, crystal orientation was analyzed, inwhich measuring results having an average CI value of 0.3 or more wereused to improve reliability of the analysis.

Analytical results (boundary map) were obtained assuming that a regionenclosed by a boundary line having difference in azimuth of 10 degreesor more by analysis of the bcc-Fe crystal orientation was the “grain” inthe embodiment of the invention. In the obtained boundary map, an areaof an individual region (crystal unit) enclosed by the boundary line wasobtained using the image analysis software “Image-Pro” (manufactured byADVANSOFT Ltd.), then circle equivalent diameter (diameter) wascalculated from the area as the grain diameter of an individual grain.The measurement was performed for at least three samples, and theaverage grain diameter (D_(ave)) as the number average diameter, and themaximum grain diameter (D_(max)) were calculated based on allmeasurement data. Results are shown in Tables 3-1 to 3-3.

[C_(max)/C₀]

C_(max) or C₀ was measured by a combustion infrared absorption methodusing a powdered sample taken from the position of depth of D/2 or D/4,respectively. Values of C_(max)/C₀ calculated using the C_(max) and C₀are shown in Tables 3-1 to 3-3.

[Wire Drawing]

Obtained wire rods were descaled by pickling, then applied with surfacecoating by bonderizing, and then subjected to dry wire drawing. First,in wire drawing 1, wire drawing was performed under a condition of truestrain >0.25 to check presence of breakage. Furthermore, wire rods withno breakage occurring in the wire drawing 1 were subjected to wiredrawing under a further strict condition of true strain >0.50 to checkpresence of breakage. Results are shown in Tables 3-1 to 3-3.

TABLE 3-1 Grain diameter Average Maximum Diameter Hydrogen grain grainWire drawing 1 Wire drawing 2 Steel of wire content in diameter diameterFinal wire Wire Final wire Wire type Wire rod rod steel Hardness 460 ×Dave Dmax C_(max)/ diameter True drawing diameter True drawing No. No.mm ppm HV C₀ ^(0.1) μm μm C₀ mm strain result mm strain result A1 A1-112.0 2.63 383 418 6.9 23.5 1.17 10.0 0.36 X — — — A1-2 12.0 1.76 362 7.327.4 1.17 10.0 0.36 ◯ 9.0 0.58 ◯ A1-3 12.0 0.53 393 7.0 25.0 1.17 10.00.36 ◯ 9.0 0.58 ◯ A1-4 12.0 0.88 432 5.3 16.8 1.17 10.0 0.36 X — — —A1-5 16.0 2.21 349 7.3 39.0 0.98 13.0 0.42 ◯ 12.0 0.58 ◯ A1-6 16.0 1.11351 7.0 37.8 0.98 13.0 0.42 ◯ 12.0 0.58 ◯ A1-7 16.0 0.90 343 7.9 41.30.98 13.0 0.42 ◯ 12.0 0.58 ◯ A1-8 18.0 1.06 331 10.7 58.9 0.98 14.5 0.43◯ 13.5 0.58 ◯ A2 A2-1 15.0 0.40 292 420 13.5 48.5 1.03 12.0 0.45 ◯ 11.00.62 ◯ A3 A3-1 15.0 0.33 300 422 15.2 50.3 1.05 12.0 0.45 ◯ 11.0 0.62 ◯A4 A4-1 16.0 2.56 425 424 6.2 16.9 1.24 13.0 0.42 X — — — A4-2 16.0 2.42341 8.2 38.5 1.24 13.0 0.42 ◯ 12.0 0.58 X A4-3 16.0 2.26 350 8.0 39.01.24 13.0 0.42 ◯ 12.0 0.58 X A4-4 16.0 1.23 409 6.8 20.5 1.24 13.0 0.42◯ 12.0 0.58 X A4-5 11.5 1.12 303 24.3 88.3 1.10 10.0 0.28 ◯ 8.5 0.60 XA4-6 11.5 1.70 355 6.4 22.5 1.10 10.0 0.28 ◯ 8.0 0.73 ◯ Wire drawingresult ◯: no breakage, X: breakage

TABLE 3-2 Grain diameter Average Maximum Diameter Hydrogen grain grainWire drawing 1 Wire drawing 2 Steel of wire content in diameter diameterFinal wire Wire Final wire Wire type Wire rod rod steel Hardness 460 ×Dave Dmax C_(max)/ diameter True drawing diameter True drawing No. No.mm ppm HV C₀ ^(0.1) μm μm C₀ mm strain result mm strain result A5 A5-115.5 2.68 432 427 5.8 12.1 1.07 13.0 0.35 X — — — A5-2 15.5 2.53 430 6.512.7 1.07 13.0 0.35 X — — — A5-3 15.5 2.20 349 17.0 81.0 1.07 13.0 0.35◯ 11.5 0.60 X A5-4 15.5 1.75 346 8.1 42.2 1.07 13.0 0.35 ◯ 11.5 0.60 ◯A5-5 15.5 1.21 337 10.5 52.0 1.07 13.0 0.35 ◯ 11.5 0.60 ◯ A6 A6-1 15.52.68 359 429 7.2 21.4 1.01 13.0 0.35 X — — — A6-2 15.5 1.07 367 7.0 27.41.01 13.0 0.35 ◯ 11.5 0.60 ◯ A7 A7-1 15.5 2.71 393 429 7.1 23.4 1.111.30 0.35 X — — — A7-2 15.5 1.22 412 7.5 18.2 1.11 13.0 0.35 X — — — A8A8-1 14.5 2.61 352 429 12.6 61.0 1.05 12.0 0.38 X — — — A8-2 14.5 0.41341 13.5 63.9 1.05 12.0 0.38 ◯ 11.0 0.55 ◯ A9 A9-1 14.5 2.59 355 42914.0 58.4 1.10 12.0 0.38 X — — — A9-2 14.5 0.68 362 15.4 58.0 1.10 12.00.38 ◯ 11.0 0.55 ◯ A10 A10-1 14.0 0.52 352 430 8.0 53.1 1.02 12.0 0.31 ◯10.0 0.67 ◯ A11 A11-1 14.0 0.63 358 431 8.5 53.7 1.02 12.0 0.31 ◯ 10.00.67 ◯

TABLE 3-3 Grain diameter Average Maximum Diameter Hydrogen grain grainWire drawing 1 Wire drawing 2 Steel of wire content in diameter diameterFinal wire Wire Final wire Wire type Wire rod rod steel Hardness 460 ×Dave Dmax C_(max)/ diameter True drawing diameter True drawing No. No.mm ppm HV C₀ ^(0.1) μm μm C₀ mm strain result mm strain result A12 A10-113.0 0.42 343 433 9.2 59.1 1.05 11.0 0.33 ◯ 10.0 0.52 ◯ A13 A13-1 15.00.34 329 433 9.8 50.2 1.08 13.0 0.29 ◯ 11.5 0.53 ◯ A13-2 15.0 0.45 3507.7 39.4 1.08 13.0 0.29 ◯ 11.5 0.53 ◯ A13-3 15.0 0.50 402 5.3 30.3 1.0813.0 0.29 ◯ 11.5 0.53 ◯ A14 A14-1 13.0 0.29 346 435 7.6 48.9 1.05 11.00.33 ◯ 10.0 0.52 ◯ A15 A15-1 13.0 0.44 359 436 7.0 47.7 1.04 11.0 0.33 X— — — A16 A16-1 13.0 0.48 373 438 8.1 42.0 1.04 11.0 0.33 X — — — A17A17-1 12.5 2.72 359 438 8.5 30.9 1.12 11.0 0.26 X — — — A17-2 12.5 2.52372 8.3 31.3 1.12 11.0 0.26 X — — — A17-3 12.5 1.43 360 8.0 35.2 1.1211.0 0.26 ◯ 9.0 0.66 ◯ A17-4 13.0 1.33 449 8.5 16.7 1.12 11.0 0.33 X — —— A17-5 13.0 0.50 407 9.1 25.3 1.12 11.0 0.33 ◯ 9.5 0.63 ◯ A17-6 13.00.17 392 8.3 30.1 1.12 11.0 0.33 ◯ 9.5 0.63 ◯ A17-7 13.0 0.01 331 7.838.6 1.12 11.0 0.33 ◯ 9.5 0.63 ◯ A18 A18-1 13.0 0.08 350 439 7.0 33.81.12 11.0 0.33 ◯ 9.5 0.63 ◯ A19 A19-1 8.0 0.54 370 444 8.8 30.5 1.40 7.00.27 X — — — A20 A20-1 8.0 0.60 382 450 8.0 50.1 1.04 7.0 0.27 X — — —Wire drawing result ◯: no breakage, X: breakage

From the results shown in Tables 3-1 to 3-3, while breakage occurredeven in the wire drawing 1 under easy conditions in wire rods that doesnot satisfy one of the requirements of the component, the content ofhydrogen in steel, and hardness specified in the embodiment of theinvention; however, breakage did not occur in the wire drawing 1 in wirerods that satisfy all of such requirements. Furthermore, among the wirerods of the embodiment of the invention, in wire rods that satisfy therequirements of grain diameter (D_(ave) and D_(max)) and segregation ofC (C_(max)/C₀), breakage did not occur even in the wire drawing 2 understrict conditions.

The invention claimed is:
 1. A hot-rolled wire rod, comprising Fe; and,based on percent by mass: C: 0.35 to 0.65%; Si: 1.6 to 3.0%; Mn: 0.10 to1.0%; Cr: 0.1 to 2.0%; P: a positive amount of 0.025% or less; S: apositive amount of 0.025% or less; N: a positive amount of 0.006% orless; Al: a positive amount of 0.1% or less; O: a positive amount of0.0030% or less; and at least one selected from the group consisting ofMg: 0.1 to 50 ppm; and Ca: 0.1 to 50 ppm; optionally at least oneselected from the group consisting of Ni: a positive amount of 1% orless and Cu: a positive amount of 1.0% or less, optionally at least oneselected from the group consisting of V: a positive amount of 0.30% orless; Ti: a positive amount of 0.10% or less; Nb: a positive amount of0.1% or less; and Zr: a positive amount of 0.10% or less, optionally Mo:a positive amount of 1.0% or less, optionally B: a positive amount of 50ppm or less, and optionally at least one rare earth element: a positiveamount of 1.5 ppm or less, wherein the rod has a wire diameter of 9.0 to25 mm, breakage of the rod does not occur in heavy wire drawingperformed under a true strain of 0.42 or more, a content of hydrogen inthe rod is 2.50 ppm by mass or less, a hardness HV of the rod is 460×C₀^(0.1) or less, where C₀ represents a content of C measured in percentby mass in a position of depth of D/4 with D being a diameter of therod, a maximum value of hardness in the range from 331 HV to 425 HV, andan average grain diameter D_(ave) is 20 μm or less, and a maximum graindiameter D_(max) is 80 μm or less in a bcc-Fe grain of a metallographicstructure of the rod.
 2. The hot-rolled wire rod according to claim 1,satisfying:C _(max) /C ₀≦1.20 wherein C_(max) represents a content of C measured inpercent by mass in a position of depth of D/2.
 3. The hot-rolled wirerod according to claim 1, wherein the hot-rolled wire rod is obtained bya process comprising heat treating a billet at a temperature of from 500to 730° C. for 60 min or more prior to hot rolling the billet, and thebillet has a composition comprising Fe; and, based on percent by mass:C: 0.35 to 0.65%; Si: 1.6 to 3.0%; Mn: 0.10 to 1.0%; Cr: 0.1 to 2.0%; P:a positive amount of 0.025% or less; S: a positive amount of 0.025% orless; N: a positive amount of 0.006% or less; Al: a positive amount of0.1% or less; O: a positive amount of 0.0030% or less; and at least oneselected from the group consisting of Mg: 0.1 to 50 ppm; and Ca: 0.1 to50 ppm; optionally at least one selected from the group consisting ofNi: a positive amount of 1% or less and Cu: a positive amount of 1.0% orless, optionally at least one selected from the group consisting of V: apositive amount of 0.30% or less; Ti: a positive amount of 0.10% orless; Nb: a positive amount of 0.1% or less; and Zr: a positive amountof 0.10% or less, optionally Mo: a positive amount of 1.0% or less,optionally B: a positive amount of 50 ppm or less, and optionally atleast one rare earth element: a positive amount of 1.5 ppm or less. 4.The hot-rolled wire rod according to claim 1, wherein the hot-rolledwire rod is obtained by a process comprising performing a homogenizingtreatment in which a billet is held at 1250 to 1350° C. for 60 min, andthe billet has a composition comprising Fe; and, based on percent bymass: C: 0.35 to 0.65%; Si: 1.6 to 3.0%; Mn: 0.10 to 1.0%; Cr: 0.1 to2.0%; P: a positive amount of 0.025% or less; S: a positive amount of0.025% or less; N: a positive amount of 0.006% or less; Al: a positiveamount of 0.1% or less; O: a positive amount of 0.0030% or less; and atleast one selected from the group consisting of Mg: 0.1 to 50 ppm; andCa: 0.1 to 50 ppm; optionally at least one selected from the groupconsisting of Ni: a positive amount of 1% or less and Cu: a positiveamount of 1.0% or less, optionally at least one selected from the groupconsisting of V: a positive amount of 0.30% or less; Ti: a positiveamount of 0.10% or less; Nb: a positive amount of 0.1% or less; and Zr:a positive amount of 0.10% or less, optionally Mo: a positive amount of1.0% or less, optionally B: a positive amount of 50 ppm or less, andoptionally at least one rare earth element: a positive amount of 1.5 ppmor less.
 5. The hot-rolled wire rod according to claim 1, comprisingfrom 0.35 to 0.44% of C.
 6. The hot-rolled wire rod according to claim1, wherein the maximum value of hardness is in the range from 331 HV to420 HV.
 7. The hot-rolled wire rod according to claim 1, wherein the rodhas a wire diameter of 10.0 to 18 mm.
 8. The hot-rolled wire rodaccording to claim 1, wherein the hot-rolled wire rod is obtained by aprocess comprising: heat treating a billet at a temperature of from 500to 730° C. for 60 min or more prior to hot rolling the billet to obtainthe hot-rolled rod, wherein the billet has a composition comprising Fe;and, based on percent by mass: C: 0.35 to 0.65%; Si: 1.6 to 3.0%; Mn:0.10 to 1.0%; Cr: 0.1 to 2.0%; P: a positive amount of 0.025% or less;S: a positive amount of 0.025% or less; N: a positive amount of 0.006%or less; Al: a positive amount of 0.1% or less; and O: a positive amountof 0.0030% or less; and at least one selected from the group consistingof Mg: 0.1 to 50 ppm; and Ca: 0.1 to 50 ppm; optionally at least oneselected from the group consisting of Ni: a positive amount of 1% orless and Cu: a positive amount of 1.0% or less, optionally at least oneselected from the group consisting of V: a positive amount of 0.30% orless; Ti: a positive amount of 0.10% or less; Nb: a positive amount of0.1% or less; and Zr: a positive amount of 0.10% or less, optionally Mo:a positive amount of 1.0% or less, optionally B: a positive amount of 50ppm or less, and optionally at least one rare earth element: a positiveamount of 1.5 ppm or less; placing the hot-rolled rod on a cooling bedat a coiling temperature TL of 1020° C. or less to obtain a wire; andcooling the wire at an average cooling rate CR1 of 2° C./sec or morefrom the coiling temperature TL to 730° C., and at an average coolingrate CR2 of 5° C./sec or less from the coiling temperature TL to 500° C.9. The hot-rolled wire rod according to claim 1, wherein the averagegrain diameter D_(ave) is in a range of from 5.3 to 20 μm.
 10. Thehot-rolled wire rod according to claim 1, comprising Cr: 0.54 to 2.0%.11. The hot-rolled wire rod according to claim 1, comprising Cr: 0.70 to2.0%.